Severe Thunderstorm Warning 51, NWS Philadelphia/Mount Holly

Area Forecast Discussion
National Weather Service Mount Holly NJ
646 PM EDT Wed May 20 2026

.WHAT HAS CHANGED...
Heat advisory has been cancelled.

&&

.KEY MESSAGES...
1. Severe Thunderstorm Watch in effect until 9 PM this evening
for our entire area.

2. Significantly cooler temperatures and several rounds of
showers for the end of this week and the holiday weekend.

&&

.DISCUSSION...

KEY MESSAGE 1...Severe Thunderstorm Watch in effect until 9 PM
this evening for our entire area.

A strong cold front will settle across our area late this
afternoon and this evening before stalling near Delmarva into
Thursday. Showers and thunderstorms are expected ahead of and
with the front. Instability continues to increase with MLCAPE
values of 1000-1500 J/kg and SBCAPE of 1500-2500 J/kg. This will
help to intensify some of the convection as it arrives from the
west. The deep-layer shear is on the marginal side though
(strongest shear to our northwest) as are the mid level lapse
rates, however a hot well-mixed boundary layer is resulting in
rather steep low-level lapse rates (DCAPE values up to 1300
J/kg!). The combination of the steep low-level lapse rates and
any stronger/organized convective cores should locally enhance
the damaging wind (downburst) threat.

Despite the lower shear, convection should still organize some
given ample instability and the hot air mass in place. Some
clustering may tend to occur with some guidance also hinting at
some bowing segments. The primary severe weather threat from
thunderstorms should be damaging downburst winds given the low-
level inverted-V profile in the model soundings. These soundings
also show that cloud tops could extend upwards of 40,000 feet
therefore some hail is possible. Much of our region is within a
level 2 out of 5 from SPC (Slight Risk) for severe
thunderstorms. The main timeframe looks to be between 4 PM to 9
PM. General rainfall totals are 0.10-0.50 inches, however given
precipitable water values of 1.5-2.0 inches, localized higher
amounts are likely with any stronger showers or thunderstorms.
Any thunderstorms should end by late this evening, with some
showers lingering in the wake of the cold front. It will turn
noticeably cooler tonight as the winds becoming northerly.

KEY MESSAGE 2...Significantly cooler temperatures and several
rounds of showers for the end of this week and the holiday
weekend.

A cold front will settle and stall to our south as we get into
the holiday weekend. Some ripples of energy ride along and north
of it and this looks to bring some rounds of showers to our
region. A more organized surface low well to our west may
provide an increase in showers or a period of rain later Friday
and Saturday. The precipitation may become enhanced some as
surface high pressure to our northeast wedges southwestward into
our area creating some cool air damming. Furthermore, with PWAT
values around 1.50-1.90 inches over the weekend, which is
around the climatological maximum for this time of the year,
there is at least a marginal chance for excessive rainfall on
Saturday. We will be keeping an eye on how strong the baroclinic
forcing gets along the stalled boundary wedged between the high
to the northeast and the low to the west. Northeast flow along
with several rounds of showers will all result in much cooler
temperatures Thursday through Saturday, then some warming should
take place Sunday and into Memorial Day. The pattern looks
active though and therefore some additional showers are quite
possible right into Memorial Day. While not great timing for the
holiday weekend and unofficial start of summer, our region
needs rain as a long-term drought continues.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG,
KILG, KMIV, KACY and surrounding areas.

This afternoon...VFR overall with increasing clouds. Some
showers and thunderstorms arrive from the west between 19-22z
with areas of sub-VFR conditions. Locally gusty winds, 30-40
knots, with any stronger thunderstorms. Winds varying from west-
northwest to west-southwest 8-15 knots with some gusts to 20
knots. Low confidence on the timing details.

Tonight...A period of MVFR/IFR conditions with showers and
thunderstorms this evening, then some lingering showers may also
contribute to sub-VFR conditions overnight. Northwest winds
becoming north-northeast mainly 10 knots or less. Low
confidence.

Thursday...A period of sub-VFR conditions with some showers.
Northeast winds 8-12 knots, with some local gusts to 20 knots
especially at KACY. Low confidence.

Outlook...

Thursday Night through Monday...An unsettled pattern takes over
with periods of MVFR/IFR conditions expected with several
rounds of rain showers. Most widespread restrictions look to
come Saturday and Saturday Night though the entire period likely
will feature showers and low clouds with perhaps sporadic
stretches of VFR if ceilings lift a bit.

&&

.MARINE...
Southerly winds to 20 knots this afternoon will diminish this
evening and becoming west and northwest, then north to northeast
on Thursday. The northeast flow increases during Thursday and
with a much cooler air mass moving in the mixing should become
more efficient over the chilly ocean water. Seas will also build
during Thursday. Opted to issue a Small Craft Advisory for
Thursday afternoon for all waters except for the waters off
Monmouth County and the upper Delaware Bay. Some thunderstorms
could be strong enough this evening to warrant the issuance of
Special Marine Warnings.

Outlook...

Thursday Night through Friday...A Small Craft Advisory is in effect
for ocean zones south of Manasquan Inlet and the lower Delaware Bay
as periods of 25-30 kt winds and seas nearing 5 feet are expected.
Sub-SCA conditions expected on the upper Delaware Bay and coastal
waters north of Manasquan Inlet.

Friday Night through Monday...SCA conditions expected with seas of 4
to 7 feet and periods of 25-30 kt winds, mainly on Saturday.

Rip Currents...

On Thursday, east to northeast winds will ramp up to 15 to 20
mph with gusts up to 25 mph. There will be a southeast swell
with a period around 6 seconds and breaking waves of 1 to 3
feet. As a result, there is a MODERATE risk for the development
of dangerous rip currents for the Jersey Shore and for Delaware
Beaches.

On Friday, east to northeast winds will range from 15 to 20 mph
with 25 to 30 mph gusts. There will be a southeast swell with a
period of 10 to 13 seconds and breaking waves of 2 to 4 feet.
This should result in a HIGH risk for the development of
dangerous and lift threatening rip currents.

Although temperatures inland will mostly be in the 60s or so
Thursday through Saturday, and even in the 50s well inland on
Saturday, ocean temperatures will remain in the 50s. These cold
water temperatures can quickly cause hypothermia and physical
incapacitation to anyone suddenly immersed in the water.

For specific beach forecasts, visit weather.gov/beach/phi

&&

.CLIMATE...
More record breaking temperatures are possible today.

Records for our climate sites are listed below:

Monthly Record High Temperatures for May

Site                Record/Date
Allentown (ABE)         97/May 19, 1962
AC Airport (ACY)        99/May 29, 1969
AC Marina (55N)         95/May 31, 1895 & May 23, 1925
Georgetown (GED)        98/May 28, 1991
Mount Pocono (MPO)      94/May 22, 1911
Philadelphia (PHL)      98/May 19, 2026 (new)
Reading (RDG)           97/May 20, 1996 & May 19, 2026
Trenton (TTN)           99/May 23, 1911 & May 31, 1986
Wilmington (ILG)        98/May 10, 1895 & May 30, 1895

Monthly Record Warmest Low Temperatures for May

Site                Record/Date
Allentown (ABE)         71/May 31, 1975, May 28 & 31, 1991
AC Airport (ACY)        73/May 24, 2004 & May 19, 2017
AC Marina (55N)         73/May 30 & 31, 1987, & May 31, 1991
Georgetown (GED)        74/May 23, 1953 & May 20, 2019
Mount Pocono (MPO)      68/May 21, 1996
Philadelphia (PHL)      78/May 31, 1895
Reading (RDG)           72/May 28 & 31, 1939, May 31, 1991, & May 19, 2026
Trenton (TTN)           75/May 31, 1895
Wilmington (ILG)        75/May 30, 1895

Record High Temperatures
                         May 20
Site                Record/Year
Allentown (ABE)         92/1962 & 1996
AC Airport (ACY)        96/1996
AC Marina (55N)         93/1996
Georgetown (GED)        95/1962
Mount Pocono (MPO)      89/1996
Philadelphia (PHL)      94/1962 & 1996
Reading (RDG)           97/1996
Trenton (TTN)           94/1996
Wilmington (ILG)        96/1996

Record Warmest Low Temperatures
                         May 20
Site                Record/Year
Allentown (ABE)         66/2019
AC Airport (ACY)        68/1996(ties)
AC Marina (55N)         69/1996
Georgetown (GED)        74/2019
Mount Pocono (MPO)      61/1959
Philadelphia (PHL)      71/2019
Reading (RDG)           69/1959 & 1996
Trenton (TTN)           68/1903, 1959, & 1962
Wilmington (ILG)        70/2019

&&

.PHI WATCHES/WARNINGS/ADVISORIES...
PA...None.
NJ...None.
DE...None.
MD...None.
MARINE...Small Craft Advisory from 2 PM Thursday to 6 PM EDT Friday for
     ANZ431-451>455.

&&

DISCUSSION...Gorse/Hoeflich/po
AVIATION...Gorse/Hoeflich
MARINE...Gorse/Hoeflich/MPS

Hazardous Weather Outlook
National Weather Service Mount Holly NJ
506 AM EDT Wed May 20 2026

MDZ012-015-019-020-210915-
Kent MD-Queen Annes-Talbot-Caroline-
506 AM EDT Wed May 20 2026

This Hazardous Weather Outlook is for northeast Maryland.

.DAY ONE...Today and tonight.

The probability for widespread hazardous weather is low.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

MJL

Preliminary Local Storm Report
National Weather Service Mount Holly NJ
615 PM EDT Wed May 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0610 PM     Tstm Wnd Dmg     Medford                 39.88N 74.83W
05/20/2026                   Burlington         NJ   Emergency Mngr

            Wires down along Taunton Road and Himmelein
            Road in Medford, likely due to the outflow
            boundary. Time estimated from radar.

&&

Hoeflich

Preliminary Local Storm Report
National Weather Service Mount Holly NJ
620 PM EDT Wed May 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 PM     Flash Flood      Trout Lake              40.00N 75.35W
05/20/2026                   Delaware           PA   Dept of Highways

            Flooding on I-476 northbound 1.2 miles north
            of Exit 9: PA 3 - BROOMALL/UPPER DARBY.
            There is a lane restriction. Time given in
            report.

&&

Hoeflich

Preliminary Local Storm Report
National Weather Service Mount Holly NJ
624 PM EDT Wed May 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0450 PM     Tstm Wnd Dmg     East Greenville         40.41N 75.50W
05/20/2026                   Montgomery         PA   Trained Spotter

            Trees and wires down in East Greenville.
            Time estimated from radar.

&&

Hoeflich

Preliminary Local Storm Report
National Weather Service Mount Holly NJ
625 PM EDT Wed May 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0525 PM     Tstm Wnd Dmg     2 NNW Montgomeryville   40.27N 75.26W
05/20/2026                   Montgomery         PA   Trained Spotter

            Trees and wires down around Colmar. Time
            estimated from radar.

&&

Hoeflich

Preliminary Local Storm Report
National Weather Service Mount Holly NJ
629 PM EDT Wed May 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0557 PM     Tstm Wnd Dmg     1 ENE Hopewell          40.39N 74.75W
05/20/2026                   Mercer             NJ   Trained Spotter

            Several trees and wires down around
            Hopewell. Time estimated from radar.

&&

Hoeflich

Preliminary Local Storm Report
National Weather Service Mount Holly NJ
643 PM EDT Wed May 20 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0627 PM     Tstm Wnd Gst     Keyport                 40.44N 74.20W
05/20/2026  M62 MPH          Monmouth           NJ   Trained Spotter

            Weather station on dock.

&&